(1) Field of the Invention
This invention relates to an optical packet switching system and, more particularly, to an optical packet switching system for performing optical packet transmission.
(2) Description of the Related Art
With an increase in demand for communication by broadband services, in recent years long-distance large-capacity optical communication networks have appeared and development of high-speed large-capacity wavelength division multiplexing (WDM), in which a plurality of optical signals with different wavelengths are multiplexed into a single optical fiber, have advanced.
In addition, with the rapid spread of the Internet and an increase in the number of large-capacity contents, there have been demands for more high-speed large-capacity flexible optical communication networks. Accordingly, attention has been paid to optical packet switching as a technique for building such optical communication networks.
The optical packet switching is a technique for switching transmitted information as packets in a completely optical state. Compared with the conventional switching in which optical signals are temporarily converted into electrical signals, processing speeds are not limited by electronic processing but by light propagation delay time. Therefore, high-speed large-capacity transmission can be performed.
FIG. 24 shows a conventional switching system in which electrical switching is performed. A switching system 100 comprises input line cards 101-1 through 101-n, an electrical switch core section 102, and output line cards 103-1 through 103-n. 
The input line card 101-1 includes an optical/electrical converter (O/E) 101a and an electrical/optical converter (E/O) 101b. The same applies to the input line cards 101-2 through 101-n. The electrical switch core section 102 includes O/E's 102a-1 through 102a-n, an electrical switch 102b, and E/O's 102c-1 through 102c-n. The output line card 103-1 includes an O/E 103a and an E/O 103b. The same applies to the output line cards 103-2 through 103-n. 
When optical signals reach the input line cards 101-1 through 101-n, they are converted into electrical signals by the O/E's 101a and processes, such as address detection, are performed by, for example, processors. The electrical signals are converted again into optical signals by the E/O's 101b and are outputted to the electrical switch core section 102.
The electrical switch core section 102 converts the input optical signals into electrical signals by the O/E's 102a-1 through 102a-n, performs electrical switching by the electrical switch 102b, and converts the electrical signals into optical signals by the E/O's 102c-1 through 102c-n. The output line cards 103-1 through 103-n convert the optical signals outputted from the electrical switch core section 102 into electrical signals by the O/E's 103a, convert the electrical signals into optical signals by the E/O's 103b, and output the optical signals onto transmission lines.
FIG. 25 shows an optical packet switching system. An optical packet switching system 100a comprises input line cards 101-1 through 101-n, an optical switch core section 104, and output line cards 103-1 through 103-n. The operation of the input line cards 101-1 through 101-n is the same as that of the input line cards 101-1 through 101-n shown in FIG. 24 and the operation of the output line cards 103-1 through 103-n is the same as that of the output line cards 103-1 through 103-n shown in FIG. 24. However, the optical switch core section 104 performs switching on optical signals (optical packets having a pulse width of about 100 ns, for example) outputted from the input line cards 101-1 through 101-n without converting them into electrical signals.
The switching system 100 shown in FIG. 24 performs processes, such as an optical/electrical conversion, at switching time. Unlike the switching system 100, however, the optical packet switching system 100a performs high-speed optical switching on optical packets. By doing so, processing capability can be improved. The research and development are currently being advanced.
To perform switching on optical signals by the packet, gate switches will be used for turning on and off the optical signals. Gate switches for turning on and off optical signals by electric control are broadly classed under two types. In gate switches of one type, absorption is changed by the use of an electro-absorption effect. In gate switches of the other type, gain is changed by drive current applied to a semiconductor amplifier.
Electro-absorption gate switches have the defect of, for example, a large loss even in a transparent state. Semiconductor optical amplifiers (SOAs), being switches in which gain is changed by drive current applied to a semiconductor amplifier, have not only a function as an optical gate for turning on and off optical signals but also an amplifying function (optical signals are amplified and outputted when they are in the ON state). With such SOAs, optical signal losses are small, so attention is currently given to them as high-speed optical switching elements.
An optical switch which prevents signals from leaking out from the output ends of optical gates is proposed as a conventional optical switching technique using SOAs (see, for example, Japanese Patent Laid-Open Publication No. 2000-77769, paragraphs [0016]-[0021] and FIG. 1).
With SOAs, an extinction ratio is high and optical loss can be reduced by an amplification mechanism. (An extinction ratio is the ratio of the average of the light intensity of the signals “1” and “0” at the time of a gate being in the ON state to the average of the light intensity of the signals “1” and “0” at the time of the gate being in the OFF state. When an extinction ratio is high, the ON and OFF states of a gate can be identified clearly, a crosstalk component from other ports is small, and a bit error rate is low.) In addition, SOAs are optical elements of semiconductors, so they can be miniaturized at low cost by using semiconductor integration techniques.
FIG. 26 shows a conventional optical switch core section including SOAs. An optical switch core section 50 shown in FIG. 26 is a 4×4 optical switch (having input ports #1 through #4 and output ports #1 through #4). The optical switch core section 50 includes optical input switch sections 50-1 through 50-4 and optical output switch sections 50-5 through 50-8.
The optical input switch section 50-1 includes a branch coupler 51a and SOAs 52a-1 through 52a-4. The optical input switch section 50-2 includes a branch coupler 51b and SOAs 52b-1 through 52b-4. The optical input switch section 50-3 includes a branch coupler 51c and SOAs 52c-1 through 52c-4. The optical input switch section 50-4 includes a branch coupler 51d and SOAs 52d-1 through 52d-4. The optical output switch sections 50-5 through 50-8 include multiplexing couplers 53a through 53d respectively.
The operation of the switch will now be described. Optical packets #2, #3, and #4 are inputted to the input port #1 and switching operation is performed (the destination of an optical packet #n is an output port #n).
The branch coupler 51a makes the optical packets #2, #3, and #4 branch in four directions and outputs them to the SOAs 52a-1 through 52a-4 (that is to say, the optical packets #2, #3, and #4 are sent to the SOA 52a-1, the optical packets #2, #3, and #4 are sent to the SOA 52a-2, the optical packets #2, #3, and #4 are sent to the SOA 52a-3, and the optical packets #2, #3, and #4 are sent to the SOA 52a-4).
Each of the SOAs 52a-1 through 52a-4 performs the operation of turning on/off a gate in response to a switch control signal sent from a host control section (not shown in FIG. 26). In this example, the SOA 52a-2 goes into the ON state (the SOAs 52a-1, 52a-3, and 52a-4 are in the OFF state) at the timing at which the optical packet #2 arrives, the SOA 52a-3 goes into the ON state (the SOAs 52a-1, 52a-2, and 52a-4 are in the OFF state) at the timing at which the optical packet #3 arrives, and the SOA 52a-4 goes into the ON state (the SOAs 52a-1, 52a-2, and 52a-3 are in the OFF state) at the timing at which the optical packet #4 arrives. As a result, the optical packets #2, #3, and #4 are outputted.
The multiplexing couplers 53a through 53d included in the optical output switch sections 50-5 through 50-8, respectively, receive optical packets sent from the optical input switch sections 50-1 through 50-4 by switching, time-division-multiplex them, and output them from the output ports #1 through #4 respectively. In this example, a group of optical packets #2 are outputted from the output port #2, a group of optical packets #3 are outputted from the output port #3, and a group of optical packets #4 are outputted from the output port #4.
In the above description, the 4×4 optical packet switch is shown as an example. To realize N×N switching by using one-stage SOA structure (a single SOA is located on a path along which optical packets are sent) which is the same as that described above, N one-to-N branch couplers and N×N SOAs are located on the input port side and N N-to-one multiplexing couplers are located on the output port side.
If a large-capacity switching system with many ports is built in this way by using the conventional structure, the following problems arise. A crosstalk component from adjacent ports increases. It is difficult to obtain a desired optical signal to noise ratio (OSNR). In addition, it is difficult to locate a fault in a switching system using SOAs. These problems will now be described.
FIGS. 27A and 27B are views for describing how a crosstalk component from adjacent ports increases in the case of including many ports. FIG. 27A is a view showing the case where four SOAs are used for gating and where a small number of ports are included. FIG. 27B is a view showing the case where 128 SOAs are used for gating and where a large number of ports are included.
In FIG. 27A, output lines of SOAs g1 through g4 are connected to a multiplexing coupler 53, the SOA g2 is in the ON state, and the SOAs g1, g3, and g4 are in the OFF state. Ideally, a signal is not outputted when an SOA is in the OFF state. Practically, however, though the extinction ratio of an SOA is high, a small portion of a signal component and an amplified spontaneous emission (ASE) are outputted even when the SOA is in the OFF state. These are noise components and are combined by the multiplexing coupler 53 into a crosstalk component (leakage signal component) from adjacent ports.
The multiplexing coupler 53 multiplexes and outputs signal components s1 through s4 outputted from the SOAs g1 through g4 respectively. If the four SOAs are used for gating, the level of accumulated noise is low (that is to say, a crosstalk component from adjacent ports is small) and a signal selected by the SOA g2 can properly be identified at the output stage of the multiplexing coupler 53.
In FIG. 27B, output lines of SOAs g1 through g128 are connected to a multiplexing coupler 53, the SOA g2 is in the ON state, and the SOAs g1 and g3 through g128 are in the OFF state.
The multiplexing coupler 53 multiplexes and outputs signal components s1 through s128 outputted from the SOAs g1 through g128 respectively. If as many as 128 SOAs are included, 127 noise signals outputted from the SOAs which are in the OFF state and a signal component s2 selected by the SOA g2 are combined.
That is to say, if the conventional switching system shown in FIG. 26 includes many (128 or 256, for example) ports, noise signals outputted from many SOAs in the OFF state and a signal component selected by an SOA are combined by a multiplexing coupler. This increases a crosstalk component. As a result, it is difficult to identify the signal component selected by the SOA at the output stage of the multiplexing coupler. Therefore, a bit error rate drops significantly.
As stated above, the level of a crosstalk component from adjacent ports which is negligible in a switching system including a small number of ports is too high in a large-scale switching system including a large number of ports to neglect.
The problem of degradation in OSNR will now be described. FIG. 28 is a view for describing a degradation of an OSNR. In FIG. 28, a vertical axis indicates power and a horizontal axis indicates a frequency. An OSNR is the ratio of signal power to noise power and differential D between a peak value Pn of noise signal power and a peak value Ps of signal power can be considered as an OSNR. If the differential D is smaller than a certain value, the signal cannot be identified with accuracy.
In a small-scale switching system including a small number of ports, the number of branches by branch couplers is small and a signal level does not drop significantly. In addition, the level of accumulated noise due to SOAs is low. Accordingly, the differential D is greater than or equal to the certain value and a desired OSNR can be obtained.
In a large-scale switching system including a large number of ports, however, the number of branches by branch couplers is large and great branch loss occurs. As a result, a signal level drops significantly. In addition, the level of accumulated noise due to SOAs is high. Accordingly, the value of the differential D is small and a desired OSNR cannot be obtained.
The problem of difficulty in locating a fault in a switching system using SOAs will now be described. If switching is performed between the input port #1 and the output port #1 in the conventional optical switch core section 50 shown in FIG. 26, the SOA 52a-1 goes into the ON state. If optical output is sent from an input line card at the input port #1 and the optical packet is not received at the output port #1, there is a strong possibility that a fault has occurred in the SOA 52a-1. When a fault occurs in the conventional switching system having one-stage SOA structure, it is easy to locate the fault.
It is assumed that a large-scale switching system including a large number of ports is built by multistage-connecting SOAs on paths along which optical packets are sent. In such a system in which the multistage connection of SOAs is made, it is impossible to locate a fault by the above simple method.
In a large-scale switching system, n SOAs are placed on a path over which optical packets are sent. Even if an optical packet is not received at an output port, it is impossible to locate one of the n SOAs in which a fault has occurred.
In such a system, a fault may be located by monitoring output from each SOA. However, this method cannot be applied. The reason for this is as follows.
In ordinary optical line switching systems, optical power is monitored to supervise the state of a signal. FIG. 29 shows the operation for monitoring optical power. A laser diode (LD) 111 outputs an optical signal. A photodiode (PD) 113 monitors the intensity of an optical signal which a coupler 112 makes branch thereto. A monitoring result is sent to the LD 111. The LD 111 outputs an optical signal while adjusting the intensity by feedback on the basis of the monitoring result (the LD may be replaced by an optical amplifier). In the conventional optical line switching system, the state of an optical signal can be supervised in this way by monitoring it with the PD, so the operating state of the LD (or optical amplifier) can be recognized from a monitoring result.
However, such a state supervision mechanism cannot be applied in the same way to optical packet switching systems including multistage-connected SOAs. That is to say, output from an SOA with which the LD is replaced cannot be monitored with a PD.
The reason for this is as follows. The SOA performs the high-speed gating of optical packets having a pulse width of about several hundred nanoseconds. Accordingly, if optical packet signals sent at a high speed are supervised with a PD the response characteristics of which depend on a time constant in a circuit, definite pulse intensity cannot be recognized (the amplitude of a waveform actually observed is approximately zero).
Therefore, the ordinary state supervision mechanism using a PD cannot be applied to an SOA. A good number of SOAs are used in a large-scale system including many ports. If some fault occurs in an SOA in such a switching system, there has conventionally been no effective fault locating mechanism. This makes it very difficult to locate the SOA in which the fault has occurred, resulting in degradation in reliability.
As has been described in the foregoing, SOAs are considered to be suitable as main optical devices for high-speed optical packet switching techniques. In conventional switching systems using SOAs, however, as their scale increases, the above problems become more marked and transmission quality is degraded. Accordingly, in the field of optical communication it is greatly hoped that an optical packet switching system in which a crosstalk component is reduced, in which an OSNR is improved, and in which a fault is efficiently detected will be realized.